1. Field of the Invention
The present invention relates to a noise elimination apparatus for use in an image processing apparatus such as a copying machine or facsimile.
2. Description of the Prior Art
In recent years, noise elimination has been an indispensable scheme for improving an image quality in the field of image processing. Particularly lately, recycle paper has been increasingly used for the protection of environment in the field of image processing of static images in a copying machine, facsimile, and printer, which leads to a growing need for eliminating noise contained in recycle paper. For instance, when a great amount of documents produced with recycle paper is subject to copying, noise contained in the recycle paper is also undesirably copied to result in image quality deterioration. In the above-mentioned case, supposing that a document is copied onto recycle paper, the resulting copy on the recycle paper is destined to have noise approximately doubled. In other words, the more recycle papers mediate in copying, the more noise increases to further deteriorate image quality.
Furthermore, for a variety of reasons other than the noise in recycle paper, noises are included in image data. For instance, dust on a platen cover in a copying machine causes a high-density noise to deteriorate an image quality.
In order to give solution to the above-mentioned problems, there have been several noise eliminating methods of smoothing an image by means of a smoothing filter, removing an isolated pixel by means of a scanning window, and appropriately setting a noise cut level as disclosed in Japanese Patent Laid-Open Publication HEI-1-160173. According to the method disclosed in the Japanese Patent Laid-Open Publication HEI-1-160173, a histogram of an image signal is firstly obtained, and then a hill of the histogram with respect to the background is detected. Thereafter, a background noise cut level is set around a right or left foot portion of the hill on the side of a signal level containing image data to eliminate noise.
According to the conventional method of smoothing an image by means of a smoothing filter, a noise component occupying a great area exerts significant influence on pixels other than an attentional pixel (core pixel for processing), the noise elimination processing cannot be performed effectively and character edges blur.
According to the method of removing an isolated pixel by means of a scanning window, a noise component having a size greater than that of the scanning window cannot be removed and yet necessary pixels may be undesirably eliminated.
According to the method of setting a noise cut level, a noise component which has a density lower than the densities of necessary image components of a document as located on the background of the document is only eliminated. In other words, there is a problem that any noise component of which density is higher than the background noise cut level is not eliminated.